Composite cathode active material having improved power characteristics, and secondary battery, battery module, and battery pack including the same

ABSTRACT

Provided is a composite cathode active material including layered lithium manganese oxide and lithium-containing metal oxide. 
     Also, the present invention provides a secondary battery, a battery module, and a battery pack which have improved power characteristics by including the composite cathode active material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/KR2012/010575 filed Dec. 6,2012 , which claims the priority of Korean Patent Application No.10-2011-0130207 filed on Dec. 7, 2011 , in the Korean IntellectualProperty Office, the disclosures of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a composite cathode active materialhaving improved power characteristics, and a secondary battery, abattery module, and a battery pack which include the same.

BACKGROUND ART

Recently, lithium secondary batteries have been used in various fieldsincluding portable electronic devices such as mobile phones, personaldigital assistants (PDAs), and laptop computers. In particular, in linewith growing concerns about environmental issues, research into lithiumsecondary batteries having high energy density and discharge voltage asa power source of an electric vehicle able to replace vehicles usingfossil fuels such as gasoline vehicle and diesel vehicle, one of majorcauses of air pollution, have been actively conducted and some of theresearch are in a commercialization stage.

Meanwhile, in order to use a lithium secondary battery as a power sourceof the electric vehicle, the lithium secondary battery must maintainstable power in a wide state of charge (SOC) range along with highpower.

An electric vehicle is classified as a typical electric vehicle (EV),battery electric vehicle (BEV), hybrid electric vehicle (HEV), orplug-in hybrid electric vehicle (PHEV) according to a type of the powersource thereof.

However, since the typical electric vehicle (EV) and the series-typePHEV among the foregoing electric vehicles entirely depend on thebattery rather than the engine in terms of driving of the vehicle,maintaining of stable power according to battery characteristics in ausable SOC range is a very important factor for driving safety incomparison to other types of electric vehicles, and a battery having awide available SOC range is required.

Meanwhile, with respect to LiCoO₂, a typical cathode material of ahigh-capacity lithium secondary battery, practical limits of an increasein energy density and power characteristics have been reached. Inparticular, when LiCoO₂ is used in high energy density applications,oxygen in a structure of LiCoO₂ is discharged along with structuraldegeneration in a high-temperature charged state due to its structuralinstability to generate an exothermic reaction with an electrolyte in abattery and thus, it becomes a main cause of battery explosion.

In order to improve the safety limitation of LiCoO₂, uses oflithium-containing manganese oxides, such as layered crystal structureLiMnO₂ and spinel crystal structure LiMn₂O₄, and lithium-containingnickel oxide (LiNiO₂) have been considered, and a great deal of researchinto layered structure lithium manganese oxides, in which manganese (Mn)as an essential transition metal is added in an amount greater thanthose of other transition metals (excluding lithium) to layered lithiummanganese oxide as a high-capacity material, has recently beenconducted.

The lithium manganese oxide exhibits relatively large capacity and alsoexhibits relatively high power characteristics in a high SOC range.However, resistance may rapidly increase at an operating voltage limit,i.e., a low SOC range, and thus, power may rapidly decrease and initialirreversible capacity may be large.

Since there are such limitations in using typically known cathode activematerials of lithium secondary batteries alone, use of a mixture formedtherebetween may be required. In particular, in order to be used as apower source of medium and large sized devices, there is an urgent needfor a lithium secondary battery having safety improved by exhibiting auniform profile without a rapid voltage drop in an entire SOC range aswell as having high capacity.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention provides a composite cathode active materialhaving improved power characteristics in which stable power may beprovided in a wide SOC range by mixing lithium manganese oxide andlithium-containing metal oxide having a high discharge potential.

The present invention also provides a secondary battery, a batterymodule, and a battery pack which include the composite cathode activematerial.

Technical Solution

According to an aspect of the present invention, there is provided acomposite cathode active material including: a layered lithium manganeseoxide represented by Chemical Formula 1 below; and a lithium-containingmetal oxide represented by Chemical Formula 3 below,a[Li₂MnO₃].(1−a)[LiM¹O₂]  Chemical Formula 1wherein, 0<a<1 and M¹ is one or more metals selected from the groupconsisting of manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe),chromium (Cr), vanadium (V), copper (Cu), zinc (Zn), titanium (Ti),aluminum (Al), magnesium (Mg), and boron (B); andLi(Ni_(x)Co_(y)Al_(z))O₂  Chemical Formula 3wherein, x, y, and z are each independently selected from a range of0<x, y, z<1, and x+y+z=1.

A mixing ratio of the layered lithium manganese oxide represented byChemical Formula 1 and the lithium-containing metal oxide represented byChemical Formula 3 may be in a weight ratio ranging from 99:1 to 50:50.

M¹ in Chemical Formula 1 of the layered lithium manganese oxide mayinclude Mn, Ni, and Co.

x, y, and z in Chemical Formula 3 may be in ranges of 0.45 to 0.90, 0.05to 0.35, and 0.005 to 0.20, respectively.

The composite cathode active material may further include one or moreselected from the group consisting of lithium cobalt oxide; lithiumnickel oxide; lithium manganese oxide; lithium cobalt-nickel oxide;lithium cobalt-manganese oxide; lithium manganese-nickel oxide; lithiumcobalt-nickel-manganese oxide; and oxides having one or more otherelements selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr,V, Ti, Cu, B, calcium (Ca), Zn, zirconium (Zr), niobium (Nb), molybdenum(Mo), strontium (Sr), antimony (Sb), tungsten (W), and bismuth (Bi)substituted or doped therein.

According to another aspect of the present invention, there is provideda secondary battery including the composite cathode active material.

According to another aspect of the present invention, there is provideda battery module including the secondary battery.

According to another aspect of the present invention, there is provideda battery pack including the battery module.

The battery pack may be used as a power source of one or more medium andlarge sized devices selected from the group consisting of an electricvehicle (EV) including an electric car, a hybrid electric vehicle (HEV),and a plug-in hybrid electric vehicle (PHEV); an electric bike; anelectric scooter; an electric golf cart; an electric truck; and anelectric commercial vehicle.

Advantageous Effects

A composite cathode active material of the present invention may notonly be non-toxic and economy in terms of costs, but may also providestable power in a wide state of charge (SOC) range.

Also, power characteristics may be improved by including the compositecathode active material according to the present invention in asecondary battery, a battery module, and a battery pack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing power with respect to each state of charge(SOC) of cells according to Examples 1 to 3 and Comparative Example 1;

FIG. 2 is a graph showing resistance with respect to each SOC of thecells according to Examples 1 to 3 and Comparative Example 1; and

FIG. 3 is a graph showing charge and discharge curves of the cellsaccording to Example 3 and Comparative Example 1.

MODE FOR CARRYING OUT THE INVENTION

A composite cathode active material according to an embodiment of thepresent invention includes layered lithium manganese oxide representedby the following Chemical Formula and lithium-containing metal oxiderepresented by the following Chemical Formula 3:a[Li₂MnO₃].(1−a)[LiM¹O₂]  Chemical Formula 1wherein, 0<a<1 and M¹ is one or more metals selected from the groupconsisting of manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe),chromium (Cr), vanadium (V), copper (Cu), zinc (Zn), titanium (Ti),aluminum (Al), magnesium (Mg), and boron (B); andLi(Ni_(x)Co_(y)Al_(z))O₂  Chemical Formula 3wherein, x, y, and z are each independently selected from a range of0<x, y, z<1, and x+y+z=1.

Since the composite cathode active material according to the embodimentof the present invention includes the layered lithium manganese oxiderepresented by Chemical Formula 1 and the lithium-containing metal oxiderepresented by Chemical Formula 3 , the composite cathode activematerial may not only be non-toxic and relatively inexpensive incomparison to LiCoO₂, but may also provide stable power in a wide stateof charge (SOC) range.

However, in the case that the layered lithium manganese oxiderepresented by Chemical Formula 1 is used alone as a cathode activematerial, a secondary battery including the layered lithium manganeseoxide exhibits a phenomenon in which powder decreases due to a rapidincrease in resistance at a low SOC. Various explanations related tothis phenomenon have been proposed, but generally, the phenomenon may beexplained as below. That is, as expressed in the following reactionformulae, two lithium ions and two electrons are extracted with oxygengas from Li₂MnO₃ constituting the layered lithium manganese oxide duringinitial charging at a high voltage of 4.5 V or more based on a cathodepotential, but only one lithium ion and one electron are reversiblyinserted into a cathode during discharging.(Charging) Li₂Mn⁴⁺O₃→2Li⁺+2e ⁻+½O₂Mn⁴⁺O₂(Discharging) Mn⁴⁺O₂+Li⁺ +e ⁻→LiMn³⁺O₂

Therefore, initial charge and discharge efficiency of the layeredlithium manganese oxide represented by Chemical Formula 1 may bedifferent according to a content of Li₂MnO₃ (value of a), but may belower than that of a typical layered cathode active material, e.g.,LiCoO₂, LiMn_(0.5)Ni_(0.5)O₂, or LiMn_(0.33)Ni_(0.33)Co_(0.33)O₂.

In this case, since a capacity of an anode must be over-designed inorder to prevent precipitation of lithium at the anode during an initialcycle according to a large irreversible capacity of the layered lithiummanganese oxide represented by Chemical Formula 1, an actual reversiblecapacity may be decreased. Also, with respect to a layered material,there may be limitations in terms of safety.

Therefore, in the present invention, the composite cathode activematerial including both the layered lithium manganese oxide representedby Chemical Formula 1 and the lithium-containing metal oxide representedby Chemical Formula 3 is provided, in order to not only address theforegoing limitations and maintain advantages of the layered lithiummanganese oxide represented by Chemical Formula 1, but also to securestable power in a wide SOC range.

According to an embodiment of the present invention, M¹ in ChemicalFormula 1 may include Mn, Ni, and Co in order to provide economicadvantages and a high-capacity secondary battery.

According to an embodiment of the present invention, a compound having adischarge potential higher than that of the layered lithium manganeseoxide may be used as the lithium-containing metal oxide represented byChemical Formula 3 in order to be mixed with the layered lithiummanganese oxide represented by Chemical Formula 1 to overall increasethe discharge potential.

That is, with respect to the composite cathode active material includingthe layered lithium manganese oxide represented by Chemical Formula 1and the lithium-containing metal oxide represented by Chemical Formula 3having a high discharge potential, since the discharge potential thereofmay be overall increased in comparison to the case in which the layeredlithium manganese oxide represented by Chemical Formula 1 is used alone,power may be stable in a wide SOC range and a phenomenon, in whichresistance rapidly increases at a low SOC, may be mitigated.

More particularly, the discharge potential of the layered lithiummanganese oxide represented by Chemical Formula 1 may be differentaccording to a type of metal selected as M¹ of Chemical Formula 1, butmay be about 3.5 V on the average. Therefore, the discharge potential ofthe lithium-containing metal oxide represented by Chemical Formula 3 maybe 3.5 V or more, but the discharge potential thereof is not limitedthereto.

In the composite cathode active material according to the embodiment ofthe present invention, a mixing ratio of the layered lithium manganeseoxide represented by Chemical Formula 1 and the lithium-containing metaloxide represented by Chemical Formula 3 may be in a weight ratio rangingfrom 99:1 to 50:50. However, in the case that the lithium-containingmetal oxide is mixed in a weight ratio of less than 1, stable power maynot be secured in a wide SOC range, and in the case that thelithium-containing metal oxide is mixed in a weight ratio of greaterthan 50, an excessive amount of the lithium-containing metal oxide moreexpensive than the layered lithium manganese oxide is included and thus,the composite cathode active material may not be suitable for alarge-capacity device in terms of costs.

A metal oxide represented by the following Chemical Formula 2 may beused as the lithium-containing metal oxide according to the presentinvention, but the present invention is not limited thereto.LiM²O₂  Chemical Formula 2wherein, M² is one or more metals selected from the group consisting ofMn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

In the present invention, the lithium-containing metal oxide representedby Chemical Formula 3, in which M² is Ni, Co, and Al among metal oxidesrepresented by Chemical Formula 2, may be used.

According to an embodiment of the present invention, x, y, and z inChemical Formula 3 may be in ranges of 0.45 to 0.90, 0.05 to 0.35, and0.005 to 0.20, respectively. In the foregoing ranges, a dischargepotential is in a range of 3 V to 4.4 V, sufficiently higher incomparison to that of the layered lithium manganese oxide represented byChemical Formula 1.

One or more compounds selected from the group consisting of lithiumcobalt oxide; lithium nickel oxide; lithium manganese oxide; lithiumcobalt-nickel oxide; lithium cobalt-manganese oxide; lithiummanganese-nickel oxide; lithium cobalt-nickel-manganese oxide; andoxides having other element(s) substituted or doped therein may befurther mixed in the composite cathode active material according to theembodiment of the present invention as needed.

The other element(s) may be one or more selected from the groupconsisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, calcium (Ca), Zn,zirconium (Zr), niobium (Nb), molybdenum (Mo), strontium (Sr), antimony(Sb), tungsten (W), and bismuth (Bi), but the other element(s) is notlimited thereto.

The present invention also provides a secondary battery including theforegoing composite cathode active material.

A typical method known in the art may be used for a method of includingthe composite cathode active material according to the embodiment of thepresent invention in the secondary battery and provision of thesecondary battery including the composite cathode active material willbe described as a non-restrictive example below.

The secondary battery includes a cathode, an anode, a separator, and anelectrolyte, and the cathode includes the cathode active materialaccording to the embodiment of the present invention.

A cathode current collector may be coated with a cathode slurry obtainedby dispersing the composite cathode active material, a conductive agent,and a binder in a dispersion medium and then, dried to prepare a cathodeincluding the composite cathode active material.

Any conductive agent, binder, dispersion medium, and current collectorknown in the art may be used so long as the conductive agent, thebinder, the dispersion medium, and the current collector are used inlithium secondary batteries.

Non-restrictive examples of the conductive agent may be one or moreselected from the group consisting of artificial graphite, naturalgraphite, carbon black, acetylene black, Ketjen black, Denka black,thermal black, channel black, carbon fibers, metal fibers, aluminum,tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium,chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver,gold, lanthanum, ruthenium, platinum, iridium, titanium oxide,polyaniline, polythiophene, polyacetylene, and polypyrrole.

The conductive agent may be used in an amount ranging from 3 parts byweight to 20 parts by weight based on 100 parts by weight of thecomposite cathode active material. In the case that a content of theconductive agent is less than 5 parts by weight, electrochemicalproperties of the battery may deteriorate and in the case that thecontent thereof is greater than 20 parts by weight, energy density perweight may decrease.

Non-restrictive examples of the binder may be one or more selected fromthe group consisting of polyvinylidene fluoride (PVdF), apolyhexafluoropropylene-polyvinylidene fluoride copolymer (PVdF/HFP),poly(vinyl acetate), polyvinyl alcohol, polyethylene oxide,polyvinylpyrrolidone, alkylated polyethylene oxide, polyvinyl ether,poly(methyl methacrylate), poly(ethyl acrylate), polytetrafluoroethylene(PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, astyrene-butadiene rubber, an acrylonitrile-butadiene rubber, anethylene-propylene-diene monomer (EPDM) and a mixture thereof.

The binder may be used in an amount ranging from 3 parts by weight to 15parts by weight based on 100 parts by weight of the composite cathodeactive material. In the case that a content of the binder is less than 3parts by weight, adhesion between the electrode active material and thecurrent collector may be insufficient, and in the case that the contentof the binder is greater than 15 parts by weight, adhesion may be good,but a capacity of the battery may be decreased because a content of theelectrode active material is decreased as much as the content of thebinder.

Non-restrictive examples of the dispersion medium may beN-methylpyrrolidone, dimethyl formamide (DMF), dimethyl sulfoxide(DMSO), ethanol, isopropanol, water, and a mixture thereof.

Non-restrictive examples of the cathode current collector may beplatinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag),ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al),molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W),indium (In) doped SnO₂ (ITO), fluorine (F) doped SnO₂ (FTO), and analloy thereof, or stainless steel surface treated with C, Ni, Ti, or Ag.

An anode, a separator, and an electrolyte used in the secondary batterymay be used without limitation so long as the anode, the separator, andthe electrolyte are used in the art.

Specifically, the anode may be prepared by coating an anode currentcollector with an anode active material, such as lithium metal, lithiumalloys, amorphous carbon, crystalline carbon, carbon composites, andSnO₂, and then drying and rolling the anode current collector.

More particularly, alloys of lithium with metals, such as aluminum,zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, orindium, may be used as the lithium alloys.

As the anode current collector, Pt, Au, Pd, Ir, Ag, Ru, Ni, STS, Cu, Mo,Cr, C, Ti, W, ITO, FTO, and an alloy thereof may be used, and Cu orstainless steel surface treated with C, Ni, Ti, or Ag may be also used.

Both of the foregoing cathode current collector and the anode currentcollector may have a form, such as a foil, a film, a sheet, a punchedbody, a porous body, or a foamed body.

The separator acts to prevent a short circuit between the cathode andthe anode, and to provide a movement path of lithium ions. A knownmaterial used in a secondary battery may be used as the separator.Non-restrictive examples of the separator used may be layers ofpolyolefin-based polymer, such as polypropylene and polyethylene, ormultilayers, microporous films, woven fabrics, and nonwoven fabricsthereof.

Carbonate, ester, ether, or ketone may be used alone or in combinationas the electrolyte. Dimethyl carbonate, diethyl carbonate, dipropylcarbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethylcarbonate, ethylene carbonate, propylene carbonate, or butylenecarbonate may be used as the carbonate, γ-butyrolactone, n-methylacetate, n-ethyl acetate, or n-propyl acetate may be used as the ester,and dibutyl ether may be used as the ether. However, the presentinvention is not limited thereto.

A lithium salt may be further added in the electrolyte. A lithium saltknown in the art may be used as the lithium salt, but non-restrictiveexamples of the lithium salt may be LiBF₄, LiPF₆, LiAsF₆, LiSbF₆, orLiPF₃(CF₂CF₃)₃.

Also, the present invention provides a battery module includingsecondary batteries including the composite cathode active material byconnecting the secondary batteries in series or parallel according to atypical technique of the art.

The number of the secondary batteries included in the battery module maybe adjusted in consideration of application and capacity of the batterymodule.

Further, the present invention provides a battery pack in which thebattery modules are electrically connected.

The battery pack may be used as a power source of a medium and largesized device, such as an electric vehicle (EV) including an electriccar, a hybrid electric vehicle (HEV), and a plug-in hybrid electricvehicle (PHEV); an electric two-wheeled vehicle including an electricbike and an electric scooter; an electric golf cart; an electric truck;and an electric commercial vehicle, but the present invention is notlimited thereto.

Hereinafter, the present invention will be described in more detail,according to specific examples. However, the present invention is notlimited thereto.

EXAMPLE 1

90 parts by weight of(0.5)[Li₂MnO₃].(0.5)[Li(Ni_(0.33)Co_(0.33)Mn_(0.33)O₂)] and 10 parts byweight of Li(Ni_(0.80)Co_(0.15)Al_(0.05)O₂) were mixed to prepare acomposite cathode active material.

Thereafter, 100 parts by weight of the composite cathode activematerial, 5 parts by weight of carbon black as a conductive agent, and 5parts by weight of polyvinylidene fluoride as a binder were added into1-methyl-2-pyrrolidone to prepare a slurry for preparing a cathode.

Next, an aluminum foil was coated with the slurry and then dried toprepare a cathode. The formation of the cathode was performed by hotrolling at 110° C. and drying in a vacuum oven at 80° C. for 24 hours.

A porous polyethylene separator was disposed between the cathode thusprepared and a graphite-based anode, and a lithium electrolyte wasinjected to prepare a polymer-type lithium secondary battery.

EXAMPLE 2

A polymer-type lithium secondary battery was prepared in the same manneras Example 1 except that 80 parts by weight of(0.5)[Li₂MnO₃].(0.5)[Li(Ni_(0.33)Co_(0.33)Mn_(0.33)O₂)] and 20 parts byweight of Li(Ni_(0.80)Co_(0.15)Al_(0.05)O₂) were mixed as a compositecathode active material.

EXAMPLE 3

A polymer-type lithium secondary battery was prepared in the same manneras Example 1 except that 70 parts by weight of(0.5)[Li₂MnO₃].(0.5)[Li(Ni_(0.33)Co_(0.33)Mn_(0.33)O₂)] and 30 parts byweight of Li(Ni_(0.80)Co_(0.15)Al_(0.05)O₂) were mixed as a compositecathode active material.

COMPARATIVE EXAMPLE 1

A polymer-type lithium secondary battery was prepared in the same manneras Example 1 except that 100 parts by weight of(0.5)[Li₂MnO₃].(0.5)[Li(Ni_(0.33)Co_(0.33)Mn_(0.33)O₂)] was used insteadof a composite cathode active material.

EXPERIMENTAL EXAMPLES

-   (1) Power Evaluation According to State of Charge

Powers according to states of charge (SOCs) of the polymer-type lithiumsecondary batteries prepared according to Examples 1 to 3 andComparative Example 1 were measured, and the results thereof arepresented in FIG. 1.

-   (2) Resistance Measurement According to State of Charge

Resistances according to SOCs of the polymer-type lithium secondarybatteries prepared according to Examples 1 to 3 and Comparative Example1 were measured, and the results thereof are presented in FIG. 2.

-   (3) Charge and Discharge Potential Measurement

Charge and discharge potentials of the polymer-type lithium secondarybatteries prepared according to Example 3 and Comparative Example 1 weremeasured, and the results thereof are presented in FIG. 3.

Data presented in the following FIGS. 1 through 3 were merely examplesand the specific values thereof may be changed according tospecifications of cells and surrounding environments, and thus, relativetrends of power, resistance, and charge and discharge potentialaccording to a SOC of each cell may be considered to be important ratherthan the specific values thereof.

Referring to FIG. 1, with respect to Comparative Example 1, powermeasured was overall lower than those of Examples 1 to 3. In the casethat a SOC is lower than 50%, the power tends to be noticeably decreasedand particularly, it may be understood that differences in the powers ofthe cells according to Comparative Example 1 and Examples 1 to 3 were10% or more in a SOC range of 20% to 40%.

Referring to FIG. 2, with respect to Comparative Example 1, a resistancevalue measured was overall greater than those of Examples 1 to 3, andparticularly, with respect to Comparative Example 1, it may be confirmedthat resistance was rapidly increased in the case that the SOC was lowin a range of 10% to 40%. However, with respect to Examples 1 to 3, itmay be confirmed that resistances were not rapidly increased as inComparative Example 1 and the resistance values thereof were also lowerthan that of Comparative Example 1.

Referring to FIG. 3, a discharge graph of Comparative Example 1 wasdisposed under that of Example 3 and thus, it may be understood that adischarge potential of the cell including the composite cathode activematerial of Example 3 was higher than that of the cell including thesingle cathode active material of Comparative Example 1.

According to the Examples and the Experimental Examples, resistances ofthe cells including the composite cathode active material according tothe present invention may not be rapidly increased at a low SOC, andthus, it may be confirmed that stable power may be secured in a wide SOCrange.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims. The preferred embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

Industrial Applicability

A composite cathode active material of the present invention may notonly be non-toxic and economy in terms of costs, but may also providestable power in a wide SOC range. Also, power characteristics may beimproved by using the composite cathode active material according to thepresent invention in a secondary battery, a battery module, and abattery pack.

The invention claimed is:
 1. A composite cathode active material comprising: a layered lithium manganese oxide represented by Chemical Formula 1 shown below; and a lithium-containing metal oxide represented by Chemical Formula 3 shown below, a[Li₂MnO₃].(1−a)[LiM¹O₂]  Chemical Formula 1 wherein, 0<a<1 and M¹ is one or more metals selected from the group consisting of manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe), chromium (Cr), vanadium (V), copper (Cu), zinc (Zn), titanium (Ti), aluminum (Al), magnesium (Mg), and boron (B); and Li(Ni_(x)Co_(y)Al_(z))O₂  Chemical Formula 3 wherein, x, y, and z are each independently selected from a range of 0<x, y, z<1, and x+y+z=1.
 2. The composite cathode active material of claim 1, wherein a mixing ratio of the layered lithium manganese oxide represented by Chemical Formula 1 and the lithium-containing metal oxide represented by Chemical Formula 3 is in a weight ratio ranging from 99:1 to 50:50.
 3. The composite cathode active material of claim 1, wherein M¹ in Chemical Formula 1 of the layered lithium manganese oxide includes Mn, Ni, and Co.
 4. The composite cathode active material of claim 1, wherein x, y, and z in Chemical Formula 3 are in ranges of 0.45 to 0.90, 0.05 to 0.35, and 0.005 to 0.20, respectively.
 5. The composite cathode active material of claim 1, further comprising one or more selected from the group consisting of lithium cobalt oxide; lithium nickel oxide; lithium manganese oxide; lithium cobalt-nickel oxide; lithium cobalt-manganese oxide; lithium manganese-nickel oxide; lithium cobalt-nickel-manganese oxide; and oxides having one or more other elements selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, calcium (Ca), Zn, zirconium (Zr), niobium (Nb), molybdenum (Mo), strontium (Sr), antimony (Sb), tungsten (W), and bismuth (Bi) substituted or doped therein.
 6. A secondary battery comprising the composite cathode active material of claim
 1. 7. A battery module comprising the secondary battery of claim
 6. 8. A battery pack comprising the battery module of claim
 7. 9. The battery pack of claim 8, wherein the battery pack is used as a power source of one or more medium and large sized devices selected from the group consisting of an electric vehicle (EV) including an electric car, a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV); an electric bike; an electric scooter; an electric golf cart; an electric truck; and an electric commercial vehicle. 